As is generally known, microwave technology provides a potential for comparatively long wireless communication range as compared to other technologies like magnetic cards, bar coded cards, inductive cards and medium wave cards.
A number of microwave transponders are known that have a directed lobe diagram, i e with a sensitivity that is high on one side while the other side has a low sensitivity. To communicate these transponders, it is thus necessary to orient them with the sensitive side towards their read- or read/write equipment, hereafter called interrogator, to obtain a secure and long range communication.
Especially concerning personal identification and data communication, it would be convenient if it wasn't necessary to orient the card with a special side towards the interrogator's antenna. This is especially true in cases when the card is hand carried such as at a time recording terminal, a teller machine or paying automate. Ultimately, no special education of the user would be necessary if the only orientation requirement for secure communication is that the transponder is held close to the interrogator's antenna.
At the same time, it would be desirable if the transponder could be made to function securely also when it, e.g. in the form of an identification card, is carried close to the human body, such as in a card holder. The requirement for long communication distance is extra pronounced when the card this way for instance is carried fixed to the front pocket of a jacked or to the waist band of a skirt, since the interrogator's antenna may at the same time be situated above a door or under a ceiling.
Different solutions to the problem of transferring information between an interrogator and a card transponder, no matter of which side of the transponder that faces the interrogator's antenna, have been presented in patent applications EP 8900147-9 and JP 63-292734.
The former describes a transponder with a dipole antenna that has equal sensitivity from front and back. A major disadvantage with this solution is however that it cannot be located close to a reflecting surface such as the human body or a card holder in a reflective material, since microwave signals are then cancelled out by reasons that are described in the literature about dipole antennas close to a ground plane.
The latter application describes a solution with separate antennas for reception and reradiation, at both sides of the card, i e in total using up to four antennas. This solution is comparatively size consuming and also expensive to implement in a product.